The requirements for cleaner fuel oil are an increasingly important challenge for petroleum processing. Crude oil fractions are processed by being "cracked" in a refinery by passing such fractions through a catalytic cracker, followed by fractionation in a distillation column. Fluid Catalytic Crackers (FCC) units include a fluidized bed reactor and a regenerator. The reactors are vessels containing a finely divided catalyst. Incoming petroleum feed stocks are generally vaporized by contact with heated catalyst and pass as a stream of mainly gas through the reactor at a sufficient velocity to maintain the catalyst particles in the form of a fluidized bed. The cracked feed stock passes from the catalyst bed through cyclone separators or dust collectors, which retrieve the bulk of the catalyst particles through the use of a centrifugal flow pattern, and then into a fractionating column or system. A fraction of the spent catalyst is discharged into the regenerator where accumulated carbon is burned from the particles at high temperatures. Generally the type of cracker employed depends on the type of feed stock, such as a gas oil cracker for fractionating light oils, and a residual oil cracker for fractionating heavy oils and tar.
A commonly used fluidized bed catalytic cracker is one which employs a zeolite catalyst in the form of alumina-silicate base particles. In this and other systems, small particles of catalyst or "fines" become entrained in the fluid stream passing through the cracker and are not separated by the cyclones, and as a result enter the fractionating system. Most of the entrained catalyst fines are retained in the heaviest fraction leaving the main column of the fractionator. This fraction is referred to as main column bottoms (MCB) or as fluidized catalytic cracker bottoms (FCCB), or as bottoms slurry oil.
Several alternative apparatuses have been considered for removing catalyst contaminants from the bottoms slurry oil by workers in the petroleum industry. Hydrocyclones were considered, but since these work best at lower viscosities they necessarily must operate at higher temperatures than is considered practical or safe. Hydrocyclones also have a removal efficiency of only about 70%. Conventional filters were also considered, but it was found in trial runs that such filters became plugged and it was not practical to clean them by backflushing. An apparatus which has been found to successfully clean slurry oil is a separator which operates by passing the oil to be cleaned through a bed of glass beads maintained in an electrostatic field. This separator is referred to herein as an electrostatic bead bed separator, and acts to capture contaminating particles as the oil passes through the void spaces surrounding the bead surfaces. Such separators are easily backflushed with compatible oils or solvents as the beads become saturated with contaminants. These electrostatic bead bed separators have proved to be efficient in removing catalyst particles from oils and can be efficiently backflushed for cleaning.
This electrostatic bead bed separator is described in U.S. Pat. No. 3,928,158 to Fritsche et al. The principles of bead bed purification as described in this patent have been adapted to large-scale commercial use in petroleum refining, in a commercial unit called the Gulftronic.TM. separator, sold by General Atomics in San Diego, Calif.
The Gulftronic.TM. separator employs glass beads of high resistivity, such as soda-lime glass, having a resistivity of 6.2.times.10.sup.8 l ohm-cm at 125.degree. C. The electrostatic bead beds employing these beads are effective in removing particulate contaminants, in particular pieces of catalyst, with as high as 95% efficiency. However, new requirements for cleaner oils having less than 100 parts per million (ppm) (by weight), and in some cases having 5 ppm or even less of contaminants, prompted a search for materials which could provide even more efficient separation to purify oils up to 99% or even essentially 100% free from catalyst particles and other contaminants.
Furthermore, it has been found that during operation of an electrostatic separator or filter, such as the Gulftronic.TM. separator, sodium ion depletion of the bead surface is observed over time. This results in weakening and cracking of the beads, and also results in changes in the electrical conductivity of the beads which require adjustments in operating conditions.
Therefore, it has become desirable to find beads which would provide an improved performance when placed under an electrical field for separation of oils from contaminants.